Go on a Tuesday or Sunday, much quieter. The locals come out in force on the Friday night parties.
The MK closes at 6PM. If you are already in the MK, you will be directed to Tomorrowland. If you aren't, then you will be held at the gates. Tip : Your MNSSHP ticket will gain you admission to the park after 4PM (or they have in years past) without other admission.
Between 6PM and 7PM they put up the decorations and change the lighting. On slower nights I found they got this done more quickly, and was often let into the rest of the park by 6:30, but don't count on it.
Most attractions are open (Hall of Presidents isn't, possibly Tiki Room as well), many more than on old "E-Ride" nights. There is additional entertainment : ghost stories in Liberty Square (the CM who does them in costume is brilliant), the special "Boo To You!" parade, which features some villians you don't generally see, a special version of the fireworks, the riverboat ride gets "haunted", and of course, trick or treating stations.
All of it is great fun, but I'd totally advise against bothering with the trick or treating. The lines are often the longest - longer than any attraction open that night. And all that is given out is generic, store-bought mini-candy bars (hersheys, etc) and tootsie pops. Doesn't even SAY disney on it - it's kind of silly to watch people stand in line for 20 minutes to get a tootsie roll they can get a bag full of at home for $2, LOL.
Avoid Fantasyland early in the evening - the kiddos come out en masse and the line for things like Peter Pan is very long. Also, the Haunted Mansion post-fireworks usually has long lines. But except for those, the lines are generally minimal, especially after 10PM. I did Splash Mountain nine times one night between 10:30 and 11:45.
MNSSHP is great fun. I'm planning on doing at least two this October - the MK is very special at night, and the darker atmosphere (though nothing "scary") really is a treat.
Good luck!
AEfx